1. Field of the Invention
The present invention relates to a photoelectric conversion device, an image sensor, and a method of manufacturing a photoelectric conversion device.
2. Description of the Related Art
Up to now, a CMOS image sensor is constructed from a photoelectric conversion device, in which a large number of photoelectric conversion elements are arranged as pixels, and a CMOS circuit for processing signal charges from the photoelectric conversion device, both provided on a semiconductor substrate such as a silicon substrate (see, for example, JP 2002-170945 A).
Each of the pixels of the photoelectric conversion device is made by: a semiconductor layer of a first conductivity type (for example, P-type) which is formed on a semiconductor substrate of a first conductivity type (for example, P+-type) and which has a lower first conductivity type impurity concentration than the semiconductor substrate; and a photoelectric conversion region (for example, N-type region) which is formed as a photoelectric conversion element of a second conductivity type in an upper side position of the first conductivity type layer. That is, the semiconductor substrate and the photoelectric conversion region construct a photodiode serving as a pixel. The semiconductor layer of the first conductivity type is made from an epitaxial layer.
In the case of the above-mentioned structure, annihilation of electrons which diffuse toward the semiconductor substrate in a part of photoelectrons generated in the first conductivity type layer takes place through recombination in the semiconductor substrate and can contribute to reduction in crosstalk of electrical charge to adjacent pixels and an image lag in which photoelectrons reach the same pixel with a time difference equal to or longer than a predetermined time.
The CMOS circuit includes, for example, a first conductivity type or second conductivity type MOS transistor. In particular, when the first conductivity type MOS transistor (for example, PMOS transistor) is to be provided to the semiconductor substrate, a second conductivity type well (for example, N-well) which reaches from a surface of the first conductivity type layer to a surface of the semiconductor substrate is formed.
However, since the semiconductor layer formed on the semiconductor substrate is an epitaxial layer, a manufacturing cost for the photoelectric conversion device becomes higher.
Formation of a PN junction of the second conductivity type well of the MOS transistor with the semiconductor substrate having a higher first conductivity type impurity concentration than the first conductivity type semiconductor layer results in a relatively high impurity concentration of the PN junction, which reduces an electrical withstand voltage of the second conductivity type well.